1. Field of the Invention
The invention relates to an analysis method for a sulfur component using ultraviolet fluorescence adopted for measuring concentrations of sulfur components including sulfur dioxide (hereinafter referred to as SO2) in a sample gas such as, for example, an automobile emission, and an analysis apparatus used in conducting the method.
2. Background Art
A nondispersive infrared gas analysis method (NDIR method) has conventionally been adopted in general aspects as a measuring method for concentrations of sulfur components such as SO2 in a sample gas such as an automobile emission. In the NDIR method, an inconvenience could not be avoided even if an optical filter is used, because of a low removal percentage of interferential components such as H2O and HC coexisting with a sulfur component as a measurement objective in a sample gas, the interferential components give an adverse influence on a measurement precision.
In order to eliminate the adverse influence caused by such interferential components (which is hereinafter simply referred to as interferential influence), a necessity arises for installing a greatly expensive pretreatment facility, leading to complexity and scaling-up of the apparatus in the entirety in which H2O is removed using a Perma-pure drier (PPD) and HC is removed by burning it to convert to CO2 in a high temperature combustion furnace at a temperature in the vicinity of 900° C. with an oxidation catalyst.
Even in a case where such a pretreatment facility is installed, the interferential influence cannot be perfectly removed. Therefore, in addition to the pretreatment facility, another necessity arises for eliminating the interferential influence, for example, by adopting a so-called fluid modulation method (cross-flow) in which a sample gas and a reference gas are alternately introduced into a sample cell at a given period in a given amount, resulting in faults due to not only further increasing a facility cost and scaling-up of the apparatus, but also lacking responsiveness to a sudden change in concentration of a gas to be measured to thereby cause a transient error and in turn, to disable high measurement precision.
As a measuring method for a sulfur component concentration as a replacement for the NDIR method having faults in facility cost and in measurement precision caused by responsiveness to a sudden change in concentration, an analysis method for a sulfur component based on ultraviolet fluorescence has conventionally been already known in which a sample gas is illuminated with ultraviolet to detect an intensity of fluorescence caused by the ultraviolet illumination and measure concentrations of sulfur components including SO2 in the sample gas. Although an analysis method for a sulfur component using ultraviolet fluorescence has a higher ability in selectively measuring only sulfur components such as SO2 to be measured as compared with other analysis methods such as the NDIR method, included in a sample gas is nitrogen monoxide (hereinafter referred to as NO) emitting fluorescence in a similar way to that of a sulfur component to be measured, especially SO2, and NO works as an interferential component in measurement of SO2, leading to a severe interferential influence.
In more detail, in a case of an analysis method for a sulfur component using ultraviolet fluorescence, while illumination is conducted with ultraviolet having a selected wavelength in the vicinity of 220 nm in measurement of SO2, NO included in a sample gas as an interferential component has narrow isolated absorption wavelength regions in the vicinities of wavelengths of 214 nm and 226 nm, which are extremely close to an ultraviolet wavelength for exciting SO2, and fluorescence spectra of SO2 and NO overlap each other to thereby detect an intensity of fluorescence emitted from NO together with that from SO2, resulting in a problem that NO exerts a conspicuous interferential influence on measurement on SO2, which leads to a measurement error.
As an eliminating means for the interferential influence caused by NO, there has been conventionally available a gas filter scheme in which a wavelength selecting section including a gas filter, in which NO is sealed, selectively absorbing light in the vicinities of wavelengths of 214 nm and 226 nm, is installed in an optical path between a light source emitting ultraviolet and a sample chamber of an ultraviolet fluorometric analyzer not to thereby cause NO to be excited and fluoresce (see, for example, JP Publication No. 11-183385).
A subtraction method has been proposed in which provided are: a sample chamber into which a sample gas including SO2 and NO is introduced; and a sample chamber into which a sample gas is introduced by way of a mechanism to remove SO2 with active charcoal; and in addition, two light sources emitting ultraviolet to respective both sample chambers; and two detection sections detecting fluorescence in respective both chamber, wherein a fluorescence amount detected in the latter sample chamber is subtracted from a fluorescence amount detected in the former sample chamber to thereby obtain a fluorescence amount only from SO2 including no interferential influence caused by NO because of the removal (see JP Publication No. 7-63683).
In a case of the gas filter scheme, however, among the above conventional NO interferential influence removal means, an absorption ability of a selected wavelength is reduced by chronological changes in property, condition or the like of a sealed NO gas, and the interferential influence of NO cannot be fully removed through continuous measurement over a long term, which gives birth to a possibility to cause a measurement error.
In a case of the subtraction method, not only are two sets of a sample chamber, a light source and a detection section necessary to be installed, but an arithmetic circuit operating a subtraction is also required, therefore, having led to a problem of complexity and cost-up of the apparatus in the entirety. And furthermore, it takes a considerable time in removal of the interferential influence by applying a subtraction operation on both fluorescence amounts after both fluorescence amounts detected in the two sample chambers are stored and held, having led to another problem of poor measurement efficiency for a SO2 concentration.